I'm too lazy to post screenshots so just bear with me.

WARNING: Contains end-of-the-line quest content spoilers.

For those of you who already got to the point after the massive radio tower collapses, you should know where to find Nelson. You should have noticed by now that you are limited by invisible walls in Washington; you can bypass it. Right before you meet nelson there should be one of those external fire stairs you need to climb up; you can actually crawl under it, as the wooden pallet isn't completely blocking off the hole, and you're able to fall through it and wander the whole Washington in its unfinished state. You can actually go back too: there is an area where you should be able to see some zombies; there's a couple narrow gaps on the building right next to that area, and you can crawl back in order to proceed normally.

If this is not intended (which i strongly feel it is) then maybe patch it? or leave it as is so people can admire the whole place like I did. It was truly a wonderful experience, especially the part where I accidentally figured out how to get back into the invisible wall.